1. Field of the Invention
The present invention relates to systems and methods for testing MIMO signal systems, and in particular, testing such systems using minimal test equipment.
2. Related Art
Many modern devices utilize wireless signals to send and receive data. Handheld devices in particular make use of wireless connections to provide features including telephony, digital data transfer, and geographical positioning. Although a variety of different wireless-connectivity capabilities are used (such as WiFi, WiMAX, and Bluetooth), in general each is defined by an industry-approved standard (such as IEEE 802.11, IEEE 802.16 and IEEE 802.15, respectively). In order to communicate using these wireless-connectivity capabilities, devices must adhere to the parameters and limitations specified by the associated standards.
Although differences exist between wireless communication specifications (for example, in the frequency spectra, modulation methods, and spectral power densities used to send and receive signals), almost all of the wireless connectivity standards specify the use of synchronized data packets to transmit and receive data. Furthermore, most devices adhering to these wireless communications standards employ transceivers to communicate; that is, they transmit and receive wireless radio frequency (RF) signals.
At any point along the device-development continuum, it may be necessary to test and verify that a device is operating according to the standards associated with its various communication capabilities. Specialized systems designed for testing such devices typically contain subsystems operative to communicate with a wireless communications device during a test. These subsystems are designed to test that a device is both sending and receiving signals in accordance with the appropriate standards. The subsystems must receive and analyze device-transmitted signals and send signals to the device that subscribe to industry-approved standards.
The testing environment generally consists of the device under test (DUT), the tester, and a computer/controller. The tester is generally responsible for communicating with the DUT using a particular wireless communication standard. The computer and tester work together to capture a DUT's transmitted signals and then analyze them against the specifications provided by the underlying standard to test the DUT's transmission capabilities.
In the case of DUT having multiple input/multiple output (MIMO) capabilities, this technology supports simultaneously transmitting discrete RF signals modulated using different data streams while using the same frequency channel. Ordinarily, doing so would cause the signals to interfere with each other making it impossible to decode the resulting composite signals. However, MIMO technology makes use of orthogonal carriers and exaggerated multipath effects on the combined signals received by multiple receivers, which allows the composite signals to be decoded to produce the original discrete data streams.
Ideally, testing the respective transmit performances of the transmitters in a MIMO device, one would isolate the transmitted signals, e.g., using separate coaxial transmission lines, to allow measurements of the individual transmitted signals simultaneously. In the case of a 2×2 MIMO device, for example, one would use two separate VSAs in order to test two simultaneous transmissions. The cost of such testers would therefore increase with the increased number of simultaneous signals to be measured. Accordingly, it would be desirable to find a way to provide for simultaneous measurement and testing of MIMO signals while avoiding significant increases in tester complexity and costs.